1. Field of Invention
The present invention relates generally to the braking of an internal combustion engine, specifically to engine braking apparatus integrated in the engine exhaust valve train.
2. Prior Art
It is well known in the art to employ an internal combustion engine as brake means by, in effect, converting the engine temporarily into a compressor. It is also well known that such conversion may be carried out by cutting off the fuel and opening the exhaust valve(s) at or near the end of the compression stroke of the engine piston. By allowing compressed gas (typically, air) to be released, energy absorbed by the engine to compress the gas during the compression stroke is not returned to the engine piston during the subsequent expansion or “power” stroke, but dissipated through the exhaust and radiator systems of the engine. The net result is an effective braking of the engine.
An engine brake is desirable for an internal combustion engine, particularly for a compression ignition type engine, also known as a diesel engine. Such engine offers substantially no braking when it is rotated through the drive shaft by the inertia and mass of a forward moving vehicle. As vehicle technology has advanced, its hauling capacity has increased, while at the same time rolling and wind resistances have decreased. Accordingly, there is a heightened braking need for a diesel-powered vehicle. While the normal drum or disc type wheel brakes of the vehicle are capable of absorbing a large amount of energy over a short period of time, their repeated use, for example, when operating in hilly terrain, could cause brake overheating and failure. The use of an engine brake will substantially reduce the use of the wheel brakes, minimize their wear, and obviate the danger of accidents resulting from brake failure.
There is also a desire to use an engine brake when shifting gears in the gearbox of the vehicle. This is apt to be an even more important aspect in commercial vehicles such as trucks and buses that are ever more frequently equipped with automatic or semi-automatic gearboxes. Such gearboxes can be likened to conventional manual gearboxes, with the difference being that the shifting of gears is carried out by means of a control device, instead of manually by the driver. In order to reduce loss of driving power of the engine during up-shift, it is an advantage if the engine speed can be matched to the new gear ratio as soon as possible. It is known to selectively introduce an engine brake during an up-shift when certain operating parameters are obtained, in order to achieve a rapid decrease of engine speed during the gear shifting process. In this way, it is alleged that wear on the engine brake system is decreased since the introduction of the engine brake only takes place during a small part of the total amount of the up-shift process.
There are different types of engine brakes. Typically, an engine braking operation is achieved by adding an auxiliary engine valve event called an engine braking event to the normal engine valve event. Depending on how the engine valve event is produced, an engine brake can be defined as:                (a) Type I engine brake—the engine braking event is produced by importing motions from a neighboring cam, which generates the so called Jake brake;        (b) Type II engine brake—the engine braking event is produced by altering existing cam profile, which generates a lost motion type engine brake;        (c) Type III engine brake—the engine braking event is produced by using a dedicated cam for engine braking, which generates a dedicated cam (rocker) brake;        (d) Type IV engine brake—the engine braking event is produced by modifying the existing engine valve lift, which normally generates a bleeder type engine brake;        (e) Type V engine brake—the engine braking event is produced by using a dedicated valve train for engine braking, which generates a dedicated valve (the fifth valve) engine brake.        
The engine brake can also be divided into two big categories, i.e., the compression release engine brake (CREB) and the bleeder type engine brake (BTEB). Here, the focus is the compression release engine brakes.
Conventional compression release engine brakes open the exhaust valve(s) at or near the end of the compression stroke of the engine piston (also known as top dead center or TDC). They typically include hydraulic circuits for transmitting a mechanical input to the exhaust valve(s) to be opened. Such hydraulic circuits typically include a master piston that is reciprocated in a master piston bore by a mechanical input from the engine. Hydraulic fluid in the circuit transmits the master piston motion to a slave piston in the circuit, which in turn, reciprocates in a slave piston bore in response to the flow of hydraulic fluid in the circuit. The slave piston acts either directly or indirectly on the exhaust valve(s) to be opened during the engine braking.
An example of a prior art CREB is provided by the disclosure of Cummins, U.S. Pat. No. 3,220,392, which is hereby incorporated by reference. Engine braking systems based on the patent have enjoyed great commercial success. However, the prior art engine braking system is a bolt-on accessory that fits above the overhead. In order to provide space for mounting the braking system, a spacer may be positioned between the cylinder head and the valve cover that is bolted to the spacer. This arrangement may add unnecessary height, weight, and costs to the engine. Many of the above-noted problems result from viewing the braking system as an accessory to the engine rather than as part of the engine itself.
As the market for compression release-type engine brakes (CREB) has developed and matured, there is a need for design systems that reduce the weight, size and cost of such retarding systems, and improve the inter-relation of various ancillary equipments, such as the turbocharger and the exhaust brake with the retarding system. In addition, the market for compression release engine brakes has moved from the after-market, to original equipment manufacturers. Engine manufacturers have shown an increased willingness to make design modifications to their engines that would increase the performance and reliability and broaden the operating parameters of the compression release-type engine brake.
(a) Earlier Integrated Rocker Brake
One possible solution is to integrate components of the braking system with the rest of the engine components. One attempt at integrating parts of the compression braking system is found in U.S. Pat. No. 3,367,312 to Jonson, which discloses an engine braking system including a rocker arm having a plunger, or piston, positioned in a cylinder integrally formed in one end of the rocker arm wherein the plunger can be locked in an outer position by hydraulic pressure to permit braking system operation. Jonson also discloses a spring for biasing the plunger outward from the cylinder into continuous contact with the exhaust valve to permit the cam-actuated rocker lever to operate the exhaust valve in both the power and braking modes. A control valve is used to control the flow of pressurized fluid to the rocker arm cylinder so as to permit selective switching between braking operation and normal power operation.
However, the control valve unit of Jonson's compression braking system is positioned separately from the rocker arm assembly, resulting in unnecessarily long fluid delivery passages and a longer response time. This also leads to an unnecessarily large amount of oil that must be compressed before activation of the braking system can occur, resulting in large compliance and less control over the timing of the compression braking. Moreover, the control valve is a manually operated rotary type valve requiring actuation by the driver often resulting in unreliable and inefficient braking operation. Also, rotary valves are subject to undesirable fluid leakage between the rotary valve member and its associated cylindrical bore.
(b) Integrated Rocker Brake with Two-Valve Opening for Engine Braking
Another integrated engine braking system for commercial vehicles is known from U.S. Pat. No. 5,564,385 (“the '385 patent”) in which a stroke-limited hydraulic piston is arranged at the operating end of a rocker arm for taking up valve play in the valve mechanism of the engine. A pressure regulating valve is utilized for supplying pressurized oil to the hydraulic piston for taking up valve play in the rocker arm. The oil is supplied to the rocker arm by means of a canal, which is provided with an exhaust in the shape of a very narrow hole through which oil can flow, and in this way be made to affect the valve body to, depending on operation, be positioned in any of the predetermined positions. For this purpose, the control valve is also provided with an adjustable magnet valve arranged for drainage of oil that has been fed through the narrow hole.
Although the engine brake system disclosed in the '385 patent has enjoyed considerable commercial success, it has some drawbacks. One of the drawbacks is that it includes a small and carefully defined hole for the transport of oil, which causes a high sensitivity to clogging and tolerances. In addition, this previously known valve causes a relatively slow coupling and de-coupling, which is particularly noticeable in connection with gear shifting. Also, the design is sensitive to external disturbances, for example in the form of temperature changes and pollution such as, for example, dirt particles or coatings.
Another drawback is related to the hydraulic actuation of the engine brake system, which inherits with high compliance. High compliance leads to large valve lift deflection, which leads to increased valve load. And increased valve load leads back to higher compliance. In order to reduce hydraulic compliance, the hydraulic piston must be designed with a large diameter. The large diameter hydraulic piston takes a long time to attain its extended position. Therefore the system taught by the '385 patent is not suitable for use in reducing engine speed at an up-shift.
Another problem with such prior art engine brakes is that the normal operation of the exhaust valve is affected during brake operation. Clearance between the cam follower and camshaft is effectively reduced during brake operation. This means that the first lobe on the camshaft opens the exhaust valve further than normal for the exhaust stroke during engine brake operation. In some cases it is necessary to provide recesses in the pistons so that the exhaust valves do not strike the pistons when the brake is operational. These recesses, and the abnormally extended exhaust valves, interfere with optimal engine design from the point of view of other considerations such as emission controls.
An additional disadvantage of the know arrangement is that it does not have an easy way or a proper lash adjusting means to set the valve lash.
(c) Integrated Rocker Brake with One-Valve Opening for Engine Braking
Instead of opening two exhaust valves during engine braking, U.S. Pat. No. 6,234,143 (“the '143 patent”) discloses an integrated rocker brake with one-valve opening for engine braking. An engine brake actuator is disposed in the rocker arm between the pivot point and the distal end. The rocker arm and the valve bridge of the engine are so arranged that the hydraulic or braking piston of the brake actuator is able to actuate on the inner valve near the pivot point of the rocker arm. By actuating only one exhaust valve, the engine braking load is greatly reduced.
The integrated engine brake system, however, has the following drawbacks. First, after the braking valve is lifted by the brake piston, the valve bridge is tilted and the followed normal valve actuation on both the braking valve and non-braking valve by the rocker arm is asymmetric or unbalanced. Large side load could be experienced on both valve stems or on the valve bridge guide if the bridge is guided. Second, the brake system can only fit on a particular type of engines that have the “parallel” arrangement of the rocker arm and the valve bridge.
(d) Integrated Rocker Brake with Reset Valve
U.S. Pat. No. 6,253,730 (“the '730 patent”) discloses an integrated rocker brake with a reset valve trying to avoid the asymmetric loading on the valves or the valve bridge caused by the engine braking operation as disclosed by the '143 patent. The reset valve will reset or retract the hydraulic piston in the rocker arm before the braking valve reaches its peak braking lift so that the braking valve will return back to its seat before the main valve lift event starts, and the rocker arm can act on the leveled valve bridge and open both the braking valve and the non-braking valve without any asymmetric loading.
However, resetting the braking valve lift around the compression TDC is very problematic. First, the duration and magnitude of the valve lift for engine braking is very small and even smaller for resetting. Second, the resetting happens at around the peak engine braking load and causes high pressure or large load on the reset valve. The timing for the resetting is critical. If the resetting happens too soon, there will be too much braking valve lift loss (lower lift and earlier closing) and lower braking performance. If the resetting happens too late, the braking valve will not be able to close before the main valve event starts and cause asymmetric loading. Therefore, the integrated rocker engine brake according to the '730 patent may not work well at high engine speeds when the reset duration and height is extremely small and the braking load or pressure on the reset valve is very high.
It is clear from the above description that the prior-art engine brake systems have one or more of the following drawbacks:
(a) The system can only be installed on a particular type of engines.
(b) The system has slow response (on & off) time.
(c) The system is hydraulically driven and has large compliance resulting in high braking load.
(d) The system causes asymmetric loading on valves or valve bridge guide.
(e) The system has too many parts, high complexity, and not work well at high engine speeds.
(f) The system has no easy way to set lash for engine braking valves.
(g) The system is not reliable and sensitive to external disturbances.
(h) The system affects normal engine performance (efficiency and emission).